This invention relates to a magnetic bearing device suited for supporting a body rotating at a high speed, e.g., a flywheel assembly, by contact-free support.
A flywheel assembly which is used for attitude control of an artificial satellite should have an inspection-free, semi-permanent life. Therefore, the flywheel assembly is supported by a magnetic bearing, which can realize perfect contact-free support. A prior art magnetic bearing used to this end comprises a radial support portion and an axial support portion, the radial support portion being of a passive type using a permanent magnet, the axial support portion being of an active type using control coils.
However, where the radial support portion of the magnetic bearing of the type noted is of a passive type using a permanent magnet, it is necessary to dispose a permanent magnet and a yoke, as a support element, in the axial direction of the flywheel body as a rotor between the opposite ends of the flywheel body and stationary bases facing these ends, such that a magnetic gap is formed. Therefore, when the degree of stiffness K.theta. around the orthogonal axis of the flywheel assembly is increased, the angular motion of the rotary portion is sacrificed, and the length or axial dimension of the assembly is increased. The stiffness K.theta. around the orthogonal axis is given as EQU K.theta.=1/4Kr(B.sup.2 -1/2.multidot.Ku/Kr.multidot.A.sup.2)
where Kr is the stiffness in the radial direction, A is the diameter of the bearing section, B is the axial length of the assembly, and Ku is the axial out-of-balance stiffness.
As is seen from this equation, the greater the diameter A of the bearing section the lower is the degree of stiffness K.theta.. Hence, to increase the stiffness K.theta. the diameter A of the bearing section must be made as small as possible. This means that the rotor side support elements constituting the radial support portion must be made as small as possible. Therefore, the rotor side support elements must be mounted in the neighborhood of the axis of the flywheel body, with the result that the mass of the rotor side support elements cannot be effectively used with respect to the angular motion of the flywheel body. This means that the angular motion per unit mass of the rotor section is inevitably small. Instead of increasing the diameter A of the bearing section in order to increase the degree of stiffness K.theta., consideration may, in light of the above equation, be given to increasing the axial length B. Doing so, however, only serves to increase the axial length of the entire assembly, which is undesirable from the standpoint of mounting the assembly in the artificial satellite.
In order to overcome the above drawbacks, a magnetic bearing has been proposed, in which the portion supporting the flywheel in the axial direction is of a passive type using a permanent magnet, and the portion supporting the flywheel body in the radial direction is of an active type using control coils.
FIGS. 1 and 2 schematically show this type of bearing. As is shown, a flywheel body 12, which is a shallow bottomed cylinder, is disposed in contact-free relation to the outer periphery of a base 10 such that it is rotatable relative to the base 10. A large-diameter annular magnet 14 is secured to the inner periphery of the flywheel body 12 and magnetized in the axial direction. A small-diameter annular magnet 16 is secured to the outer edge of the body 10 such that it faces the large-diameter magnet 14 coaxially. The magnet 16 is magnetized axially and in the opposite direction to the magnet 14. Annular yokes 18a and 18b are secured to the opposite pole surfaces of the magnet 14. Likewise, annular yokes 18c and 18d are secured to the opposite pole surfaces of the magnet 16. The magnetic flux produced by the magnets 14 and 16 thus flows through the magnet 14, yoke 18a, yoke 18c, magnet 16, yoke 18d, yoke 18b, and back to the magnet 14, whereby an axially passive type support is realized. Four control poles 20a to 20c arranged in the form of a cross are fixed to the inner periphery of the yoke 18d; the base 10 being constituted by the yoke 18d and the control poles 20a to 20d. Control coils 22a to 22d are wound around the control poles 20a to 20d, respectively. The radial position of the flywheel body 12 is stabilized through appropriate excitation of these coils.
The degree of stiffness K.theta. and orthogonality of the axis of the above magnetic bearing, which, unlike the radially passive type magnetic bearing noted before, is of the axially passive type, is greater the larger the bearing diameter and the smaller the gap between the yokes 18a (or 18b) and 18c (or 18d). Thus, it is possible to reduce the length, i.e., axial dimension, of the bearing and provide a flat and large-diameter flywheel assembly having a high degree of magnetic stiffness K.theta..
When the control coils 22a and 22c of this magnetic bearing are energized as shown in FIG. 2, the control fluxes produced by these coils flow through the control poles 20a and 20c, and also through the annular yoke 18b in the circumferential direction thereof. When the fluxes flow through the yoke 18b in the circumferential direction, there is a danger that the yoke will soon be magnetically saturated because of the small sectional area of the magnetic path. That is, the flux that can pass through the yoke 18b is limited, so that it is impossible to sufficiently control the radial position of the flywheel body 12, particularly, when a dynamic disturbance is received. In addition, the magnetic path through which the control fluxes flow is of a considerable length and is open to the outside. Therefore, leakage of the control flux to the outside of the flywheel assembly is liable to result. Therefore, where electronic circuits or the like are provided on the outer side of the flywheel assembly, they are liable to be adversely affected.
Since the annular magnet 14 in this magnetic bearing is secured to the flywheel body 12 as the rotor, it should be secured firmly enough so that it can function sufficiently when the flywheel body 12 is rotated at a high speed. Further, the rotational balance of the flywheel body 12 must be adjusted by taking the circumferential direction imbalance of the mass of the magnet 14 into consideration.
Further, when there are fluctuations of the flux from the magnet 14 in the circumferential direction due to fluctuations in the shape and magnetic characteristics of the magnet 14, the radial forces of attraction between the flywheel body and base are varied with the rotation of the flywheel body. In this case, it is difficult to efficiently stabilize the flywheel body with the control coils 22a to 22d.